Let me try to explain:
By default, the rocket boost is co-bound to the 'jump' key. Is it possible to co-bind it to something like 'sprint' through config, or would this require changing the code itself?
(I know it is possible to simply rebind the boost to Ctrl, but the co-bind thing is cooler and seems more 'intended')

No, it's not possible to co-bind it with something else.